The disclosed invention generally relates to optical proximity sensing systems, and is particularly directed to an optical proximity sensing system having a plurality of detectors having adjacent fields of view.
Optical sensing proximity systems are utilized, for example, in a movable vehicle, such as a missile, for detecting the presence of a target object which may be encountered while the vehicle is moving. Positive detection of a target object may be utilized for various purposes such as initiation of a predetermined process or maneuver. Generally, known optical sensing proximity systems include a transmitter having a transmit transducer, such as a laser beam, and a receiver having a receive transducer, such as an infrared detector. The transmitter and receiver typically also have respective processing circuitry. The transmitter and receiver are aligned so that the transmitter field of view and the receiver field of view overlap in a predetermined active region. If an object is located within the active region, the receiver provides an appropriate detection signal.
While the above described transmitter/receiver proximity system is fairly straightforward, actual use is not. Specifically, fog, smog, smoke dust, and other scattering and/or absorbing particulate matter (collectively "aerosol") in the atmosphere are known to present false alarms to optical proximity sensing systems. Further, such atmospheric conditions may obscure a target object.
Another consideration with known optical proximity sensing systems is their sensitivity to the reflectance of a target object. That is, a target object having a given reflectance will cause detection at a relatively different range than a target object having a greater or lesser reflectance.
U.S. Pat. No. 4,532,867, issued to Mitchell on Aug. 6, 1985, discloses a target detection system having a dual field of view. The Mitchell system dual field of view is intended to reduce the number of false alarms which would otherwise be generated by a single field of view system. However, it is believed that the Mitchell system may be susceptible to false alarms in an aerosol environment.
Briefly, the Mitchell system includes first and second sensing zones for first and second detectors, where the first sensing zone is closer to the detector carrying missile. The first and second detector outputs are amplified and provided to respective first and second peak hold circuits. The first peak hold circuit output is provided to a first comparator which has its other input coupled to a threshold reference. The second peak hold circuit output is provided to a second comparator which also receives the first peak hold circuit output. The outputs of the comparators are provided as inputs to an AND circuit which provides an output indicative of target detection.
The Mitchell system indicates the presence of a target when both of the following conditions are met: (a) the first detector output is greater than the second detector output, and (b) the first detector output is greater than the threshold reference.
In the presence of aerosol, the outputs of both detectors of the Mitchell system could be very close and false alarms would occur due to random noise so long as the first detector output exceeds the threshold reference.